Bearing Bracket with Three Ribs: Sectioning Rules and Presentation
A drawing office is detailing a cast-iron bearing bracket for a lathe. The bracket has a base 180 × 120 × 18 mm, a central hub Ø60 mm with a Ø40 H7 bore, and three stiffening ribs, each 12 mm thick, spaced at 120° around the hub, one of them pointing straight up in the front view. The client has asked for a full section through the axis of the bore to show the bore, the hub wall and the base. The trainee prepares the front view as a full section, hatches the hub, the base and the vertical rib all with 45° lines at 3 mm spacing, then adds hidden lines for the Ø40 bore and for the bolt slots in the base. The checker rejects the sheet. The client also asks how the other two ribs must appear, since the cutting plane passes through only one of them, and how the section should be labelled on the sheet.
1. Decide whether the hatching on the vertical rib is correct, and justify the rule with reference to what the section is meant to show.
Read the worked answer
The rib must not be hatched. Standard drawing practice (IS 696 / SP 46 conventions) is that ribs, webs, spokes and similar thin features cut longitudinally - that is, the cutting plane passes along their length and through their thickness - are left unhatched and shown only in outline.
The reason is representational, not decorative. A rib is thin in one direction only. If its long cut face is hatched like the solid hub wall, the reader sees a broad hatched band and infers a thick, massive wall where there is only a 12 mm stiffener. The drawing would then mislead the patternmaker and the moulder about the section modulus and the metal mass.
What the trainee must do
- Erase the hatching on the vertical rib; retain its outline where it is cut.
- Keep the hub wall and the base hatched in the same direction, at 45°, with thin lines uniformly spaced (2–4 mm) - the 3 mm spacing used is acceptable, but it must be identical for the same part.
- Note that if the same rib were cut transversely (across its thickness), it would be hatched like any other solid, because then the cut face shows the real thickness.
2. The two remaining ribs are at 120° and 240° to the vertical one and are not on a single straight cutting plane. Advise how they should be handled, and what must be stated on the drawing if the trainee rotates them into the section.
Read the worked answer
Option A - strict plane section. Show only what lies on the cutting plane: the vertical rib appears (unhatched), the other two do not appear in the section at all. This is always defensible but hides the fact that the bracket has three ribs.
Option B - aligned (rotated) section. Because the ribs are regularly spaced at 120°, the cutting plane may be bent: it follows the vertical rib, then turns at the hub axis to pass along a second rib, and again for the third. The portion cut by the bent plane is then rotated into the plane of projection before projecting, so each rib shows its true length and thickness. This is the usual solution for brackets, gear arms and similar castings.
The trainee must:
- Draw the cutting plane on the plan as a chain line with thick ends, showing the bends at the axis, and give the direction of sight by arrows.
- Rotate the offset legs about the axis into the projecting plane rather than freely translating features - otherwise the 120° spacing is falsified.
- Label the view SECTION A–A, and add a note such as RIB ROTATED INTO SECTION so the reader knows the section is not a true plane cut.
3. Evaluate the hidden lines added behind the section, and specify the correct labelling and line conventions for the section view.
Read the worked answer
Hidden lines behind a section should not be drawn. The purpose of a section is to expose the interior; the cut surfaces already show the Ø40 bore, so hidden lines for it are redundant and simply clutter the view. Standard practice omits hidden detail in sectioned views unless it is essential for manufacture and cannot be shown in another view. For the bolt slots in the base, the correct answer is either to show them in an end view or to omit them here - not to layer hidden dashed lines behind a hatched face.
Conventions the checker will look for
- Cutting-plane line: thick at the ends and at any change of direction, thin chain between; arrows at the ends show the direction of sight; letters A–A placed near the arrows.
- Title: SECTION A–A, centred below the section view.
- The section replaces the front view in its normal position - no extra view is added.
- Hatching: thin continuous lines at 45°, evenly spaced, same direction throughout the same part. If a mating part (bush, shaft) were shown, it would be hatched at 45° the other way to separate the two components.
- No dimension lines inside hatched areas; numerals must not sit on a hatch line.